check if element is in viewport - vanilla JS. Use by adding a “scroll” event listener to the window and then calling isInViewport().

in_viewport.js

// Determine if an element is in the visible viewport
function isInViewport(element) {
  var rect = element.getBoundingClientRect();
  var html = document.documentElement;
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || html.clientHeight) &&
    rect.right <= (window.innerWidth || html.clientWidth)
  );
}
The above function could be used by adding a “scroll” event listener to the window and then calling isInViewport().

you are probably the greatest human being in my perspective now, thanks

Thanks for sharing this inspiring piece of code. ;)

Here is an extended version that supports checking for full visibility in more specific areas.

function isInViewport(element, parentElement=null)
{
	var childRect = element.getBoundingClientRect();
	if (parentElement == null)
	{
		// check for full visibility in window area
		// (original version)
		var html = document.documentElement;
		return (
			childRect.top >= 0 &&
			childRect.left >= 0 &&
			childRect.bottom <= (window.innerHeight || html.clientHeight) &&
			childRect.right <= (window.innerWidth || html.clientWidth)
		);
	}
	else
	{
		// check for full visibility in a specific parent element's area
		// (for scrollable containers that don't fill the whole window)
		var parentRect = parentElement.getBoundingClientRect();
		return (
			childRect.top >= parentRect.top &&
			childRect.left >= parentRect.left &&
			childRect.bottom <= parentRect.bottom &&
			childRect.right <= parentRect.right
		);
	}
}

why arent you guys using the
IntersectionObserver API in vanilla JS?

More about it here < https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API >

The event listener on scroll will execute a function many timer as a user scrolls about while the element is in the viewport - madness with a slideshow! Also it takes at least 2 scroll events for element reach the viewport fully.

Need to hit the function only once...

let timerOn = false;

function isInViewport() {
  const ele = document.querySelector(".carousel");
  const rect = ele.getBoundingClientRect();
  if (rect.top >= 0 && rect.bottom <= window.innerHeight && rect.left >= 0 && rect.right <= window.innerWidth) {
	  if (!timerOn) timer();
  }
};

function timer() { 
  timerId = setInterval("plusSlides(1)", 3000); 
  timerOn = true;
}

window.addEventListener('scroll', function (event) {
  isInViewport();
}) ;